The present invention relates to a line for feeding strip material to a user unit, particularly a wrapping machine.
In the following description, it is assumed, purely by way of example, that the wrapping machine is of the type commonly known as "form, fill and seal."
At present, known lines for supplying a strip of wrapping material comprise a guide device defining a given path for the strip; a traction device at the output end of the path, for imparting a given traveling speed to the strip; and a tensioning device for controlling and adjusting the tension of the strip.
U.S. Pat. No. 2,710,154 relates to a pneumatic tensioning device located upstream from the traction device, and which provides for compensating any variation in the tension of the strip and so maintaining a given tension.
The above known pneumatic tensioning device comprises a cup-shaped body connected to a suction source, and presenting an open end--hereinafter referred to as a "window"--located along and defining a portion of the path along which the strip travels.
In actual use, the strip travels in airtight manner past the window, and is subjected to a substantially constant tension equal to the vacuum inside the cup-shaped body, so that a reduction in the tension applied to the strip by the traction device is immediately compensated by the formation, inside the cup-shaped body, of a loop, which gets shorter as the tension applied to the strip by the traction device is increased.
By applying a substantially constant tension to the strip, the above tensioning device is of advantage for adjusting strip tension on machines in which the strip is fed at substantially constant speed and is only rarely subjected to transient speeds and, hence, transient tensions. In fact, by applying a substantially constant tension to the strip, the above device opposes any acceleration of the strip, and acts as a brake, which, in addition to reducing transient speeds, also results in instantaneous over-tensioning and possible tearing of the strip.
As such, the above known pneumatic tensioning device is far from suitable for use on packing machines, the output speed of which is affected by numerous factors and therefore normally variable.